Platelets adhere to a site of angiopathy and are then bound (aggregated) with each other to form a thrombus, and these functions are controlled by various proteins existing on the platelet membrane. Known examples of such membrane proteins include GPI (Glycoprotein I), GPIIb (Glycoprotein IIb) and GPIIIa (Glycoprotein IIIa). With regard to GPIIIa, the full length cDNA encoding a GPIIIa protein was isolated from platelets of a normal individual, and its gene structure was specified (J. Biol. Chem. 262(9) p. 3936-3939). Further, researches on platelet antigens existing on these membrane proteins (HPA-1 to 7) have been carried out (J. Clin. Invest 40 p 1597 (1961), Prog. Hematol. 4 p 222 (1964), Vox Sang 4 p 161 (1959), Vox Sang 39 p 113 (1980)).
Platelet antigens are known as causes of neonatal alloimmune thrombocytopenic purpura (NAITP), post-transfusion purpura, refractoriness to platelet transfusion therapy, and the like. In Japanese individuals, it is said that anti-HPA-2b antibodies are involved in refractoriness to platelet transfusion therapy and anti-HPA-4-b antibodies are involved in NAITP. Thus, typing of the platelet antigen type is becoming clinically important.
Platelet antigen types occur due to polymorphisms of several glycoproteins (Table 1), which polymorphisms are yielded by a single amino acid substitution at the protein level or a single base substitution at the genetic level. Therefore, in typing of the platelet antigen based on the gene, such a difference in a single base is assessed (Japanese Journal of Transfusion Medicine. Vol. No. 1 39(1):204-211, 1993).
TABLE 1PlateletAntigenmembraneAmino acidsystemAntigenproteinsubstitutionCodonHPA-1HPA-1aGPIIIaLeu (33)CTGHPA-1bProCCGHPA-2HPA-2aGPIbThu (145)ACGHPA-2bMetATGHPA-3HPA-3aGPIIbIle (843)ATCHPA-3bSerAGCHPA-4HPA-4aGPIIIaArg (143)CGAHPA-4bGlnCAAHPA-5HPA-5aGPIaGlu (505)GAGHPA-5bLysAAGHPA-6HPA-6aGPIIIaArg (489)CGA, CGGHPA-6bGlnCAGHPA-7HPA-7aGPIIIaPro (407)CCCHPA-7bAlaGCCHPA-8HPA-8aGPIIIaArg (636)CGTHPA-8bCysTGTNakaNaka+CD36Pro (90)CCTNaka−SerTCT
However, unknown platelet antigen types also exist, and sufficient elucidation thereof has not been achieved yet. With regard to HPA-7, the only polymorphisms reported so far are HPA-7a wherein the base at position 1297 in the GPIIIa gene is cytosine (the amino acid at position 407 is proline) and HPA-7b wherein the base at position 1297 in the gene is guanine (the amino acid at position 407 is alanine) (Blood:83:70-76, 1993).